Question:
Two isolated populations M and N have population sizes of \(100\) and \(1000\) individuals, respectively. The starting allele frequencies \(p\) and \(q\) are \(0.5\) in both populations. Assuming no selection, which one of the following is expected after \(100\) generations?
Two isolated populations M and N have population sizes of \(100\) and \(1000\) individuals, respectively. The starting allele frequencies \(p\) and \(q\) are \(0.5\) in both populations. Assuming no selection, which one of the following is expected after \(100\) generations?
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Genetic drift is stronger in smaller populations, so heterozygosity (\(2pq\)) is lost faster in the smaller population M.
Updated On: Jul 20, 2026
- \(p\) is expected to be higher than \(q\) in population M but not in population N
- \(p\) is expected to be lower than \(q\) in population M but not in population N
- \(2pq\) in population M is expected to be higher than \(2pq\) in population N
- \(2pq\) in population M is expected to be lower than \(2pq\) in population N
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The Correct Option is D
Solution and Explanation
Step 1: Identify the process at work.
With no selection acting, the only force changing allele frequencies here is genetic drift, the random change in allele frequency caused by chance sampling of gametes each generation.
Step 2: Recall how drift depends on population size.
The strength of genetic drift is inversely related to population size. Small populations experience larger random swings in allele frequency each generation, while large populations experience smaller swings, because sampling error shrinks as sample size grows. Population M, with only \(100\) individuals, drifts much more strongly than population N, with \(1000\) individuals.
Step 3: Rule out options (A) and (B).
Drift has no built-in direction. Starting from \(p=q=0.5\), there is an equal chance of \(p\) drifting up or down in either population. So there is no reason to expect \(p\) to be consistently higher, or consistently lower, than \(q\) in one population and not the other. Both (A) and (B) assume a directional bias that drift does not have.
Step 4: Connect drift to heterozygosity.
The quantity \(2pq\) is the expected proportion of heterozygotes under random mating, and it also measures how much variation remains at that locus. As drift pushes allele frequencies away from \(0.5\) and eventually toward fixation (\(p=1\) or \(p=0\)) or loss, \(2pq\) shrinks toward zero.
Step 5: Compare the two populations.
Because drift is stronger in the smaller population M, allele frequencies there move away from \(0.5\) faster, and heterozygosity is lost faster. After \(100\) generations, population M is expected to have lost more of its genetic variation than the larger population N.
Step 6: Final conclusion.
So \(2pq\) is expected to be lower in M than in N.
\[ \boxed{2pq_M < 2pq_N} \]
With no selection acting, the only force changing allele frequencies here is genetic drift, the random change in allele frequency caused by chance sampling of gametes each generation.
Step 2: Recall how drift depends on population size.
The strength of genetic drift is inversely related to population size. Small populations experience larger random swings in allele frequency each generation, while large populations experience smaller swings, because sampling error shrinks as sample size grows. Population M, with only \(100\) individuals, drifts much more strongly than population N, with \(1000\) individuals.
Step 3: Rule out options (A) and (B).
Drift has no built-in direction. Starting from \(p=q=0.5\), there is an equal chance of \(p\) drifting up or down in either population. So there is no reason to expect \(p\) to be consistently higher, or consistently lower, than \(q\) in one population and not the other. Both (A) and (B) assume a directional bias that drift does not have.
Step 4: Connect drift to heterozygosity.
The quantity \(2pq\) is the expected proportion of heterozygotes under random mating, and it also measures how much variation remains at that locus. As drift pushes allele frequencies away from \(0.5\) and eventually toward fixation (\(p=1\) or \(p=0\)) or loss, \(2pq\) shrinks toward zero.
Step 5: Compare the two populations.
Because drift is stronger in the smaller population M, allele frequencies there move away from \(0.5\) faster, and heterozygosity is lost faster. After \(100\) generations, population M is expected to have lost more of its genetic variation than the larger population N.
Step 6: Final conclusion.
So \(2pq\) is expected to be lower in M than in N.
\[ \boxed{2pq_M < 2pq_N} \]
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